Cutting tools in the past were run with spears so that the cut tubular string could be retained by the spear and then pulled out of the hole. The cutter designs were variable and many included blades that extend by sliding down a ramp and turning about a pivot that was driven by a piston that was fluid driven and a spring to retract the blades. Some examples of such designs are U.S. Pat. Nos. 5,791,409; 2,136,518 and 2,167,739. Other styles for cutting tubular strings are illustrated in U.S. Pat. Nos. 7,823,632; 5,018,580; 4,856,642 and 5,014,780.
The present invention addresses several issues in the prior design and presents a more reliable and economical design. The actuating piston is flow actuated to shift and extend the cutting blades and to retain the extended blade position even after the flow is cut off. The blades are retracted with pressure on a landed plug on the piston so that a return spring is not required. The reverse movement of the piston shears out the body lock ring that had previously held the piston on the blade extended position. When the blades get through the wall of the tubular string being cut the adjacent housing squarely supports the blades that are extended radially so as to better support the cut string with reduced stress on the blades as the cut string is raised up to the point where it can be supported from slips on the rig floor so that the blades can be retracted after slacking off weight and pressuring up against a bumped plug on top of the piston. These and other features will be more readily apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be found in the appended claims.